Question

What happens when blue crystals of copper sulphate are heated?

Answer 1

Hint: When the blue crystals of copper sulphate are heated, the water of crystallization leaves the molecule. Copper sulphate is also known as blue vitriol.

Complete step by step answer:
Copper sulphate is an inorganic compound with chemical formula \[CuS{O_4}.5{H_2}O\]. The other names of copper sulphate are blue vitriol, bluestone, vitriol of copper and Roman vitriol. \[CuS{O_4}.5{H_2}O\] is a bright blue salt. It exothermically dissolves in water to give \[{[Cu{({H_2}O)_6}]^{ + 2}}\] which has octahedral molecular geometry. The structure of the solid pentahydrate is a polymeric structure where copper is again octahedral but bound to four water ligands.
When blue copper sulphate crystals are heated, the five water crystallizations of copper sulphate leave the molecule. The resultant product is a grey powder, known as anhydrous copper sulphate. The chemical reaction depicting the change is given below:
\[CuS{O_4}.5{H_2}O \to CuS{O_4} + 5{H_2}O\]
Blue copper sulphate decomposes before melting. It loses water molecules step by step. Firstly, it loses two water molecules upon heating at 63C. When it is further heated to 109C, it loses two more water molecules. The final water molecule is lost at a temperature of 200C. Dehydration proceeds by decomposition of the tetra-aquacopper(2+) moiety, two opposing aqua groups are lost to give a dia-quacopper(2+) moiety. The second dehydration step occurs when the final two aqua groups are lost. Complete dehydration occurs when the final unbound water molecule is lost. When it is heated to 650C, copper sulphate decomposes to give CuO and \[S{O_3}\].

Note: Remember that copper sulphate is found with various water crystallizations ranging from 0 to 5. But the most predominantly occurring copper sulphate is with 5% water of crystallization.